The present invention relates to a vehicle steering apparatus, and more specifically to a vehicle steering apparatus provided with a steering force assist device.
A typical vehicle steering apparatus includes a steering force assist device that applies to a steering system an assist force for assisting steering operation. An electric power steering apparatus (EPS actuator) using a motor as a drive source has been known as a steering force assist device. The steering force assist device typically executes damper compensation control based on the steering wheel turning speed, thereby improving the steering characteristic. This primarily suppresses excessive steering angle at high speed driving.
In a state of a low vehicle stability, for example, when the vehicle is traveling on a low μ road, an attempt is made to stabilize the attitude of the vehicle by performing a steering operation for applying a steering angle in a direction opposite to the direction of the yaw moment of the vehicle, that is, quick countersteering.
However, the damper compensation control is designed to attenuate the steering wheel turning speed. In other words, the damper compensation control hampers quick steering operation. Thus, quick steering operation at countersteering is influenced by the damper compensation control. Particularly, in the case of the vehicle steering apparatus (refer to Japanese Laid-Open Patent Publication No. 2005-297719), which is provided with a variable transmission ratio device, steering operation is influenced not only by the attenuation of the assist force due to the damper compensation control, but also by a reaction force torque accompanying the actuation of the variable transmission ratio device. This deteriorates the steering feel of the driver, and may hamper an appropriate countersteering by the driver.